The Miox requires you to have both batteries AND the rock salt. You run out of the salt, it doesn't work. (Although the Miox is good if you're purifying water for a large group, you can do huge batches at once, whereas the Steripen you can only do a half litre or one litre at once.)

I use the MSR Miox Purifer and I found that if you don't use too much salt the water won't have any taste at all (it takes a couple tries to figure how much you need, it's easy you just need to use as little as possible and the purifier will tell you if you didn't use enough salt). I don't think that the fact it uses salt is an issue as salt can be found anywhere and it is very cheap, plus the little bag included with the purifer last forever. Also the Miox is lighter and more compact (about half the size) than the Steripen. However Steripen is an excellent product, it is really a matter of prferences, and just so everyone know CycloCamping.com will be carrying some of their models pretty soon.

Collecting rain water that is dripping off of anything is very risky, any of the waste coming from birds, lizards or other critters will contaminate the water, Salmonella as well as other unfriendly microbes are commonly found in water from these sources.

I definitely think that you should take the filter. If you have read about the effects on the people who drink the water from the Ganges, you will filter every drop that you drink. Even if it appears that the water is fine, I would make sure to filter it. The risk is too high that you get sick or have long lasting effects from dirty water. Even at home, I make sure my refrigerator filter is changed regularly. Dirty or chemical saturated water is no good for anyone!